Another death, another boundary. This time it is a vicar who has died at the hands of a violent man. And so we add the blood of a priest to that of the London headmaster, Philip Lawrence, and the Dublin journalist, Veronica Guerin, in the catalogue of those who have died in recent times while simply doing their jobs.

With each it seems a new taboo is broken. There was a time in public imagination when non-domestic murder was something was restricted to gangland culture; our fear was contained by the supposition that, from the Krays to the Mafia or the modern warring drug-dealers, "they only killed their own kind". No longer. Our age has become so inured to the casualness of violent death that the average killing of a man in a pub brawl rates only a paragraph in the national newspapers, if it makes the news at all.

And yet there is still something profoundly shocking about certain deaths. The teacher is stabbed during his attempt to impart moral correction. The journalist is shot while fearlessly seeking after the truth. And now the priest dies in a pool of his own blood while making himself available, with great vulnerability, to the despised and rejected.

They were people of service, doing their jobs - and in an exemplary manner - so that their deaths seem more than a tragedy. They are a violation of some kind of trust, a compact whereby society brands some actions more than perpetually forbidden but marks them, in some unwritten way, as taboo. They break the limits that lead to chaos. Like the death of the children of Dunblane or a toddler such as James Bulger - open, trusting, innocent and epitomising everything we hold sacred about childhood - they strike at what society cherishes at its centre, where it needs to be most secure.

In part it is that our taboo against violence in general has been eroded. But such profanities strike at our hearts and reinforce a sense that older, better values are in terrifying collision with a new nihilism. Statistics tell us that the world is no more violent a place today than it was in previous eras; our children are statistically as safe now as they have ever been. And throughout history terrible crimes have always happened.

Yet there is more to this than moral panic. When a nurse is attacked on a ward, or a doctor on his or her rounds, or an aid worker is killed in a far-off place, our sense of shock is augmented by one of moral outrage. That someone who is not taking risks for personal gain, nor even is just a casual bystander, but who is there to help can be obliterated for no apparent reason underscores our sense of pointlessness.

The killing of a journalist is a taboo long shattered in many parts of the world. Mobsters in the United States, faction-leaders in Bosnia, drug- dealers and even governments in Latin America have wilfully murdered reporters in an attempt to silence them. But taboos are culturally specific. It has not happened in Britain and Ireland, even in the face of decades of killing in Ulster, so the gunning down of Veronica Guerin shocked both nations.

In a similar way there is nothing new about the murder of priests. Yet from Thomas a Becket onwards the killing of clerics has most often been bound up with politics or political symbolism. Men like Oscar Romero may have been motivated entirely by the gospel, but his stand in El Salvador brought him into conflict with the vested interests of the rich and powerful. The bishop blown up in Algeria recently was regarded by his Islamic assassins not as a culpable individual but as a symbol of the cultural and economic imperialism of the West.

The only killing of a clergyman in Britain in recent times was that of an elderly Catholic priest hit on the head at his home by burglars in Southwark a couple of years ago. But Fr Christopher Gray was not killed merely for being in the wrong place at the wrong time. He was killed in the mundane exercise of his ministry which was to live alongside the people who are most in need of help.

The Church of England's decision to maintain a presence in every parish in the country means that often in inner-city areas the only professional actually living in the area is the Anglican minister. The destitute, the desperate and the dangerous therefore stream to the presbytery door making life for the minister extremely trying. And yet the church is the one place where people like that can go when everyone else has rejected them.

The state deals with such people from behind the safety of the glass- sheeted social security desk. It is part of our society's claim to civilised values that there are still men like Christopher Gray whose vocation is to deal face to face with the winos and drug addicts, some of whom can be extremely violent, and attempt to comfort them with little more than a sandwich, a cup of tea and a chat.

Clergymen are not trained to do this. It is a skill they pick up in their early years as curates. Most priests become quite hard-headed about establishing the boundaries of such ministry. But if some, especially the younger and more idealistic, do not find it easy to draw the line under an unsatisfactory encounter, that only reflects the extent of the sacrifice such a calling involves. In the case of Christopher Gray - a personable young man with a brilliant academic background and great talent as a musician and linguist - it appears to have cost him his life.

No wonder David Shepherd, the Bishop of Liverpool, yesterday described the killing of Fr Gray as "an event of sheer evil". Part of the nature of evil is that it is incomprehensible. There is no making sense of it. It exists in some region beyond rational thought, where no explanation is possible.

Of course, we attempt explanations. "If we ban guns our children's lives will not have been lost in vain," say the parents of the murdered children of Dunblane. But it is a desperate attempt to barter with the unnegotiable, to make some inroads into the meaninglessness and somehow make it understandable.

The killing of anyone is evil, but where that person is working for good that evil is particularly exaggerated. It is in only cases like that of Christopher Gray that we seem to rediscover our ability, and our need, to be shocked. But with each shattered taboo the boundaries for that outrage recede.The next incident will seem somehow marginally less shocking. And we are all diminished for that.